Modern long-haul (“backbone”) and/or metropolitan (“metro”) communications networks often include one or more reconfigurable optical add-drop multiplexers (ROADMs) at geographically separated locations such as cities or distribution points within a city. By definition, a ROADM is an optical network element that routes (that is, adds or passes) optical signals directly and avoids Optical-Electrical-Optical (OEO) conversion. Integration of ROADMs into the present WDM networks provides a pay-as-you-grow capability and flexibility to provision wavelengths dynamically. This allows the ROADM to both pass through any separate wavelength between WDM node line interfaces and to add/drop any separate wavelength from WDM line interfaces to add/drop ports.
Pairs of such ROADMs are coupled by a section, which may comprise one or more optical fibers. In some examples, where the physical distance between ROADMs is long, the section may consist of a plurality of optical fiber spans, coupled in series by optical amplifiers, such as erbium doped fiber amplifiers (EDFA) and/or Raman amplifiers.
Each ROADM may be coupled to a plurality of optical and/or electro-optical communication links by which data can be added to or offloaded from the backbone network. Such links may in turn be coupled to local network nodes such as transponders, routers and/or communications devices such as computers.
The network may support thereon a plurality of wavelength channels or optical signals. Such wavelength channels may be routed through the network along a path comprising a sequence of sections coupled by ROADMs between a source and a destination node. The sequence of sections and ROADMs is known as a path. In this context, a connection for a wavelength channel refers to the path established for the wavelength channel from source to destination, together with the spectrum or wavelength assigned or allocated to the wavelength channel for each section.
It will be appreciated that “connection” is used in this fashion in the telecommunications industry and the use of such term in this description should not be taken to imply the existence of any direct physical connectivity between the source and the destination. Indeed, a “connection” will typically involve a sequence of coupled nodes and sections as discussed above.
In some examples, a section may comprise a plurality of spans of fiber coupled by amplifiers.
In some examples, adjacent fiber spans or sections in a connection for wavelength channel may have the same or different spectrum allocation. Where a connection has a common spectrum allocation throughout, the connection is said to be transparent. Where, however, the spectrum allocation is not the same throughout the connection, the connection is said to be translucent.
In some examples, a section may employ wavelength division multiplexing (WDM) to support a plurality of wavelength channels. A WDM system may differ on channel spacing and different wavelength patterns. A coarse WDM (CWDM) channel has wide spacing and fewer channels on silica fiber while a dense WDM (DWDM), following ITU recommendations, has 100 GHz or 50 GHz spacing with 40 and 80 channels across the C-band, respectively. Under ultra dense WDM, channel spacing of 12.5 GHz is possible. Furthermore, under WDM architecture there is no substantive difference between fixed-grid networks with regular wavelength spacing, such as, without limitation, 50 GHz and/or 100 GHz, and flex-grid networks with wavelength spacing that may be larger or smaller, and in some examples may be in multiples of 6.25 GHz and/or 12.5 GHz. Accordingly, in this context a wavelength channel may comprise a WDM channel, a CWDM channel, a DWDM channel, a fixed-grid channel and/or a flex-grid channel.
Typically, establishing a connection for a wavelength channel involves two substantially independent actions by different hierarchical network planes or layers.
The first action is typically performed by a path computation element (PCE) server in the control plane and involves selecting a connection for a wavelength channel from among a plurality of connection candidates. In some examples, a service layer in an application plane provides a request to the PCE server for a connection to be computed between a source node and a destination node.
Routing refers to the identification, by the PCE server using a routing algorithm, of a connection between the source and destination node for a wavelength channel.
In some examples, the PCE server is implemented as a dedicated server, and/or a PCE function as part of a server, such as a central provisioning Network Management System (NMS) server and/or distributed across a plurality of routers. In software-defined networking (SDN) systems and/or Transport SDN (T-SDN) systems, the PCE function can form part of a T-SDN controller and/or network orchestration layer and works with a PCE request and response Protocol (PCEP). The PCE server is typically presented with a request to select a connection by the service layer.
In some examples, the PCE is constrained, in terms of selecting a connection, by business considerations, including constraints imposed by any applicable service level agreements (SLA) as well as factors such as network utilization, resource efficiency, quality of service (QoS) and latency considerations.
The selection of a connection subject to such constraints typically involves routing and spectrum assignment (RSA), or, in case of fixed-spaced wavelength channels, routing and wavelength assignment (RWA). RSA is conventionally used in the context of flex-grid WDM while RWA is conventionally used in the context of fixed-grid WDM. In this disclosure, the terms RWA and RSA are used interchangeably. RSA is a subset of the PCE server and/or function that deals specifically with the photonic and/or optical layer.
In some examples, RSA may be implemented within the PCE server. RSA techniques have tended to focus on either or both of maximizing service quality (in terms of signal to noise ratio (SNR), optical SNR (OSNR) and/or Q-factor) and increasing resource utilization and/or efficiency. Examples of approaches to improve service quality include using different cost functions and/or optimization criteria, such as considering latency, number of hops and/or load balancing.
Examples of techniques to improve resource efficiency include RSA scheme such as, without limitation, fixed routing, alternative routing, shortest-path (SP), k shortest-path (k-SP) and/or A-star routing techniques.
In some examples, the selection of a connection may incur a delay on the order of several milliseconds to hundreds of milliseconds, depending upon the complexity and/or sophistication of the RSA technique employed and the available computational and storage capacity to implement the technique. Other factors also affect the overall timing for the PCE server to compute the desired connection such as, without limitation, the network size, complexity of the physical layer models and/or required accuracy.
More recently, the selection of a connection has taken into account physical layer impairment (PLI) effects on performance metrics. In this context, PLI refers to the static impairments experienced by a wavelength channel for a corresponding selection candidate. PLI can take into account post-transient effects when a traffic channel is established and the network has converged and optimized. Such PLI effects can include both linear effects and non-linear effects (e.g. optical Kerr effects) and the complex interaction between linear and non-linear effects. Linear PLI effects can include, for example, amplified spontaneous emission (ASE) of an amplifier, loss and dispersion. Non-linear PLI effects include without limitation, cross-phase modulation, self-phase modulation and four-wave mixing.
Proposals to address PLI effects have, in some examples, involved using physical layer modeling to provide estimates of PLI effects into the PCE server. Such estimates provide the RSA engine with improved information about the offered signal quality for each wavelength channel associated with a connection candidate. The estimation of PLI effects permits customized figures of merit (FOMs) such as SNR, OSNR and/or Q-factor to be defined and evaluated by the RSA engine.
In some examples, the generation of PLI effect estimates may also take on the order of several milliseconds, depending upon the accuracy and sophistication of the PLI effect estimate employed and the available computational and storage capacity to implement the estimate as well as the network size.
The second action is typically performed by a lower photonics management layer and involves the set-up of a path for a wavelength channel associated with the selected connection. In such action, provisioning information is sent to all corresponding elements involved in the establishment of the traffic, including without limitation, ROADMs and amplifiers if appropriate, to build the optical topology for the selected connection. In some examples, network provisioning can take on the order of several milliseconds, considering issues of delay latency and available computational and storage capacity at the various network elements.
Once the connection has been selected by the control plane and provisioned, the corresponding elements are subjected to physical adjustments. By way of non-limiting examples, transponders are tuned to recommended wavelengths, add/drop banks at the source and destination nodes are adjusted to pave the path for the wavelength channel set-up along the selected course, ROADMs are adjusted to open pipes, power levels are adjusted using variable optical attenuators (VOA) and the amplifiers between the ROADMs are commanded to handle a power variation. Once the physical adjustments have been made, the photonic layer effects the turn-up of the wavelength channel associated with the selected connection.
Unlike RSA, PLI and network provisioning, each of which can involve timing on the order of milliseconds, the actions to make the physical adjustments and then turn up the wavelength channel associated with the selected connection, may in some examples, take on the order of several seconds, that is, about 3 orders of magnitude longer.
In conventional backbone networks, the disparity in timing between the action of selecting a connection and the action of setting up the connection is largely irrelevant because the connection tends to be set up a priori and have very long durations. Accordingly, the actions have remained separate and effectively sequential.
In next-generation networks, it is increasingly likely that a connection will be selected and set-up on an as-needed ad hoc basis, being characterized by rapid turn-up and tear-down and short duration of the connection and its associated wavelength channel.
Methods and systems that reduce the overall set-up time of a connection through a network would be desirable.